In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with a lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide, in the exhaust port, a valve which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
United States Patent Application Publication No. 2005/0166872A1, published on Aug. 4, 2005 to Zauner and assigned to the assignee of the present application, entitled “Exhaust-Outlet Control for 2-Stroke Engine”, discloses a valve assembly having a valve provided, in part, in the exhaust port which is connected to an actuator for moving the valve between a full flow position and a flow restricting position. The actuator includes a pressure chamber defined in part by a diaphragm. The valve is connected to the diaphragm. A spring biases the diaphragm such that the valve is biased towards the flow restricting position. When a positive pressure is supplied to the pressure chamber which is sufficient to overcome the bias of the spring, the volume of the pressure chamber increases, which moves the valve to the full flow position.
U.S. Pat. No. 6,216,648, issued on Apr. 17, 2001 to Spaulding, entitled “Internal Combustion Engine With Pneumatically Controlled Variable Exhaust Valve”, discloses a valve assembly having a valve provided, in part, in the exhaust port which is connected to an actuator for moving the valve between a full flow position and a flow restricting position. The actuator includes a diaphragm located in a pressure chamber. The valve is connected to the diaphragm. By selectively supplying positive and negative pressures to the pressure chamber, the diaphragm moves in one direction or the other. Depending of which direction the diaphragm moves in, the valve moves to the corresponding full flow or flow restricting position.
Although both Zauner and Spaulding properly control the height of the exhaust port for operating at high engine speeds and at low engine speed by providing corresponding full flow and flow restricting positions, neither of them allow for reliably moving the valve to an intermediate position for operating at medium engine speeds. It may be possible to supply a pressure to the pressure chamber of Zauner which would move the valve to an intermediate position. However, for a given pressure, the volume of the pressure chamber would be different depending on the temperature. Due to the close positioning of the valve assembly relative to the engine, the temperature can vary significantly, and therefore, so would the volume of the pressure chamber. Therefore, in order to obtain the same intermediate position consistently, the pressure necessary to obtain the intermediate position would need to be consistently recalculated, which is both difficult and impractical.
Therefore, there is a need for a valve assembly suitable for use with an exhaust port of a two-stroke internal combustion engine having a valve which can be moved between a full flow position, an intermediate position, and a flow restricting position.